Due to the hyperbolic shape of the resistance formula, +50% in the Resistance when converted to damage reduction doesn't increase as the resistance goes up, but actually the increase in damage reduction decreases. For example with monster level 63 (taken from this answer):

This means that while invigorate adds 10% to health, impunity adds a smaller value to the damage reduction. This means that in the formula for effective health, invigorate replaces Health by 1.1 * Health while impunity replaces R by (R + y) where y is the increase in damage reduction (in the two examples above, 8.5%5.9% respectively).

So with invigorate, you get 10% * x bonus, while with impunity you get y * Health * (1 - A).

To get an idea which one is better, let's put numbers from my build in to see which is worth what. My base life is 50k. Resistances are at 56% reduction. Armor is at 78% reduction. 50% increase in my resistance gives about 65% reduction. This means y is 9%.

I would think the onyl time 10% health bonus out weighs the 50% resistance is if your EHP is already over 1000% and impunity only gives 300 resistance (an impossible situation since to get 1000% EHP your resistance is probably much higher in the first place.)
–
z 'Sep 9 '12 at 12:42

@yx, the link to battle.net forum from the other post was really interesting. It seems like the website I got my formula from has a different fromula than the one in that post (note that, my website is up-to-date while that post is from 3 months ago). Nevertheless, playing with impunity definitely "feels" easier, so I guess the other formula is more accurate.
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ShahbazSep 9 '12 at 17:16

EHP = Health * (A + R - AR). So, if you'd theoretically have 0 armor and 0 resistance, your EHP would be 0, even if you have 42 billion hp... In other words, you'd be dead, even if you do not get hit by a monster.
–
AlderathSep 10 '12 at 17:33

1 Answer
1

I am afraid to say the EHP formula you quoted is very wrong. EHP should be HP divided by combined damage taken rate, not multiplied by damage reduction rate. For example if you have 10k HP and 60% combined DR, which means you're only taking 40% damage, it is equivalent to having 25k HP without any damage reduction, so your EHP is 10k / (1 - 60%) = 25k.

As @yx pointed out, every 300 resistance equals to 100% EHP*, so the only case Invigorate will beat Impunity in terms of EHP increase is when 50% of your resistance is less than 30. Invigorate unlocks at lv41 and usually a character don't have much resistance (or HP) at that time, and the 310 regen could be something. However when a character reaches lv60 and enters Inferno, Impunity is the clear winner.

*Based on assumption of mLvl=60, while your DR numbers were calculated based on mLvl=63, so there might be a small gap but doesn't change the big picture.